Early aircraft speed sensors placed a pressure sensor on a rotating arm and interpreted the pressure variations resulting as a result of aircraft movement through an air mass by assuming that the pressure sensed on the rotating arm was a pure sinusoid. See, for example, U.S. Pat. No. 4,360,888 to Onksen et al and U.S. Pat. No. 3,332,282 to Daw. Other rotating pressure sensing devices are disclosed by Beilman in U.S. Pat. Nos. 3,400,584; 3,726,139; 4,065,957 and 4,074,570.
U.S. Pat. No. 4,893,261, on the other hand, assumed that the pressure samples taken from a position on the rotor were only quasi-sinusoidal and demonstrated that a performance improvement would be obtained by performing a Fourier analysis on the pressure samples. Based on the Fourier extraction and use of the steady state (DC) and first harmonic Fourier terms from the sensed total pressure, a sensed static pressure, a sensed free air stream temperature, a sensed reference indicative of the blade passing a reference position, and a sensed reference signal indicative of rotor velocity were all used to produce a more accurate determination of aircraft direction and velocity. The Fourier analysis was carried out in a discrete manner and was performed on the sensed total pressure signals using a plurality of subrevolution pressure samples that could be averaged over each subrevolution interval.
The above prior art required the direct measurement of three air data parameters: static pressure (P.sub.s), total pressure (P.sub.T) and free air stream temperature (T.sub.FAT) in order to determine the air speed velocity (V.sub.A) of an aircraft.